This invention relates to a process for forming styrenic polymer beads for producing soft molded products such as packaging material.
The formation of styrene polymer beads is a well known and commercial process. One such process is described in Grim patent U.S. Pat. No. 2,673,194, the contents of which are incorporated by reference herein. Such styrene polymer beads, upon impregnation with a blowing agent, are expanded in a continuous upward flow of steam to yield discrete pieces of foam. Then, after an aging period of several hours during which air diffuses into the expanded particles, they are placed in a slightly vented mold. The particles are then heated with steam at superatmospheric pressure injected into the mold so that they expand to fill the empty spaces and fuse together. The molding operation is completed by circulating or spraying cold water in the jacket of the mold until the foam no longer exerts pressure.
A useful property of expanded polystyrene articles is the cushioning effect that such articles provide when used as a packaging material. Attempts have been made to increase, even more, the cushioning properties of polystyrene articles, by enhancing the impact strength and resiliency of the expanded polystyrene used to form such articles. Such attempts include the polymerization of styrene (60-70 parts) after its absorption into particles (40-30 parts) of copolymerized ethylene and vinyl acetate. These particles, after impregnation with butane, yield molded foams that are softer, more resilient and have greater resistance to impact than polystyrene foams. However, because of the fugitivity of the blowing agent, the product must be expanded promptly after its production or stored in pressure-resistant containers at low temperatures.
Polyethylene foams of approximately two pounds per cubic foot density exhibit goods impact strength and resiliency but are deficient for some packaging applications because of a tendency to creep and form on enlarged cavity while bearing a relatively light load.
An object of the present invention is to provide expandable styrenic polymer beads with enhanced expandability, shelf life comparable to conventional expandable polystyrene, and utility for forming soft molded articles of low density.